To measure current flowing through an electric conductor of high-voltage in a gas insulated switch gear (hereinafter referred to as GIS) or similar apparatus, a wound-type current transformer has been conventionally used. The transformer of such type is a through-type current transformer comprised of an endless core and a secondary winding through which the electric conductor passes. However, the wound-type current transformer is large in size and heavy in weight. Because of this, consideration has been made about application of an optical current transformer that uses the Faraday effect, which is a rotation phenomenon of polarization plane of a light due to an effection of magnetic field. Such optical current transformer is suitable particularly for measuring single-phase current the measuring which will not be affected from magnetic field of the other phase.
As a usual configuration in a single-phase optical current transformer, an optical path is provided around an electrical conducting body through which current to be measured flows. With this configuration, current is measured as follows: When linearly polarized light from a light source is allowed to pass through the optical path, the effection of magnetic field of the current flowing through the electrical conducting body causes a rotation of the plane of polarization of such light because of the Faraday effect. A detector provided at the detection end of the optical path detects the Faraday rotation angle and thereby measures amount of the current.
One style of a single-phase optical current transformer can be found in for example JP 03-105259 A1 (Patent Literature 1), wherein a block of lead glass is used as the Faraday-effect element. In this single-phase optical current transformer, the optical path is formed around an electric conductor, through which current flows, by arranging assembled blocks of lead glass. Through this optical path, linearly polarized light from a light source is allowed to pass and amount of change in the Faraday rotation angle caused by the effection of magnetic field of the current in the electric conductor is detected with a detector to measure the current.
The other style of a single-phase optical current transformer can be found in for example JP 07-248338 A1 (Patent Literature 2), wherein an optical fiber is used as the Faraday-effect element. In this single-phase optical current transformer, the optical path is formed in a closed loop configuration using an optical fiber arranged encircling the electric conductor. Current measuring is performed in a similar manner to the method stated above using linearly polarized light allowed to pass through the optical path.
The optical current transformer of which optical path is fabricated using block-shaped lead glass is hard to manufacture in a smaller size and requires complicate fabrication process. Because of this, such a single-phase optical current transformer as uses an optical fiber for configuring the optical path has been considered advantageous.
In this connection, it is known that there are two types of single-phase optical current transformers having optical paths of closed loop using optical fibers; one is a reflection type and the other is a transmission type. In the reflection type, the Faraday rotation angle of the linearly polarized light passed through the optical path is detected on the returning light reflected in the optical path. In the transmission type, the Faraday rotation angle of the linearly polarized light passed through the optical path is detected on the light transmitted through the optical path.
On the other hand, GISs are becoming high in their voltage rating and, due to that, also becoming large in capacity and size. As a consequence of that, the diameter of the cylindrical container, in which the electric conductor is accommodated with insulating gas filled, has become large. Because of this, in the single-phase optical current transformer to be installed on a GIS, the optical path of closed loop using optical fiber is required to be installed outside or inside the cylindrical container of a large diameter.
Both of the conventional single-phase optical current transformers stated above have optical paths of closed loop using Faraday-effect elements. Therefore, when a large current such as fault current flows through the electric conductor, the effection of magnetic field by such heavy current will cause saturation in the size of Faraday rotation angle of the linearly polarized light propagating through the optical path. As a consequence of this, a problem arises in that the accurate measuring of current that bases on the Faraday rotation angle measured on the detector end becomes impracticable.
Further, the conventional single-phase optical current transformer has been required to have the optical path of closed loop using a Faraday-effect element in such a configuration that the path encircles the electric conductor. Therefore, there has been a problem in that, where an optical path of closed loop using optical fiber is required to be installed outside or inside the cylindrical container like in a GIS, placing such device in required position encounters a poor workability and is not easy. Moreover, temperature difference appears between the upper part and the lower part of the cylindrical container due to direct sunlight or temperature rise of the electric conductor. This temperature difference invites also temperature difference in the Faraday-effect element between its upper and lower parts connecting to the measuring error. Likewise, this problem arises even in the case where an optical fiber, which is light in weight, is used as the Faraday-effect element.
An object of the present invention is to provide a single-phase optical current transformer having features: such that saturation in the size of Faraday rotation angle of the linearly polarized light propagating through a first and a second short optical paths hardly occurs even when a heavy current such as fault current flows through an electric conductor, and therefore large electrical current in such electric conductor is measured reliably; and such that economical manufacturing in a compact size is practicable.
Another object of the present invention is to provide a single-phase optical current transformer easy to install on a part of the outer peripheral surface of a cylindrical container with eased assembling and replacing operation.